Since the 1800's fingerprint information has been collected from human fingers and hands by means of ink and paper. For the purposes of this document, the term fingerprint is used to mean the skin surface friction ridge detail of a single fingerprint, partial fingerprint or any portion of the skin surface friction ridge of up to and including the entire hand. In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and ultrasonic methods. Methods based upon ultrasound have proven to be the most accurate, since they are virtually immune to the effects of grease, dirt, paint, ink and other image contaminants.
The ultrasound method employs a piezoelectric transducer that sends a sound wave or pulse through an ultrasonic transmitting media. The pulse is then partially reflected back at each media interface. The reflected pulse is received by the transducer, and the lapsed time between sending and receiving the pulse may be used as a measure of the distance traveled by the pulse going and returning from each reflecting material interface. In order to detect those reflected signals corresponding to the fingerprint, a particular time interval may be monitored. Since the finger is positioned a known distance from the transducer and since the speed of the ultrasound signal is known, the signals reflected from the finger will be expected at the transducer during a particular time interval. This process is called range gating (biasing). The signal received during the particular time interval may be converted to a digital value representing the signal strength. The lapsed time may be displayed graphically to create a contour map of the fingerprint. Often a gray-scale bitmap image is used to graphically display the information.
Although ultrasound imaging of a fingerprint is superior in detail to a similar image collected by an optical system, it takes more time to collect a raster scanned ultrasonic image because common ultrasonic scanning mechanisms often collect each pixel of image information individually by means of a two axis mechanical scanning apparatus. Optical systems usually collect many pixels of information at a single time.